A heat-fusible wholly aromatic copolyester represented by the general formula: ##STR2## (wherein d, e, and f are not equal to O and e/f is from 0.90 to 1.10, and n is 0 or 1) has high heat resistance and exhibits excellent fluidity at suitable processing temperatures and thus, it is expected to find wide use in the fabrication of, for example, electric or electronic parts.
This wholly aromatic copolyester, however, has a disadvantage in that when injection molded, since its molding temperature is high, its melt viscosity varies markedly with even a slight change in molding temperature, and it readily undergoes orientation; thus it produces only molded articles having a bad appearance due to uneven flow marks. Further, in connection with the physical properties of its molded articles, they have a low impact strength and are brittle. Thus the wholly aromatic copolyester is difficult to put into practical use as a structural material even though it has excellent heat resistance.
In order to improve the appearance of molded articles, various attempts have heretofore been made. As the methods for improving the marked change of melt viscosity by the molding temperature, there is a method for blending with resins having a lower temperature dependency.
For example, a method of blending with polyethylene terephthalate or polycarbonate is such a method.
However, if the wholly aromatic copolyester is mixed with polyethylene terephthalate or polycarbonate, granulated, and molded in a temperature range where the wholly aromatic copolyester is uniformly melted, the polyethylene terephthalate or polycarbonate readily undergoes thermal decomposition. On the other hand, if they are treated in a temperature range where these resins can be melted stably, the fluidity of the wholly aromatic copolyester is not sufficient to enable the whole of the composition to become a uniform dispersion.
Further, as a method for increasing the impact strength of the molded articles, there is the method of blending with rubbers. In this case, however, the same problems as encountered in the blending with polyethylene terephthalate or polycarbonate occur.
Moreover, both the above-described methods are undesirable because they cause a serious reduction in heat resistance of the resulting molded articles.